The present invention relates to multilamp photoflash units and, more particularly, to a reliable sequencing circuit for compact multilamp photoflash units.
Numerous multilamp arrangements with various types of sequencing circuits have been described in the prior art; particularly in the past few years. A currently marketed photoflash unit (described in U.S. Pat. Nos. 3,894,226, 3,912,442; 3,935,442; 3,937,946; 3,941,992; 3,952,320 and 4,017,728 and referred to as a flip flash) employs high-voltage type lamps adapted to be ignited sequentially by successively applied high-voltage pulses from a source such as a camera-shutter-actuated piezoelectric element. The flip flash unit comprises an elongated planar array of eight high-voltage type flashlamps mounted on a printed circuit board with an array of respectively associated reflectors disposed therebetween. The lamps are arranged in two groups of four disposed on the upper and lower halves respectively of the rectangular-shaped circuit board. A set of terminal contacts at the lower end of the unit is provided for activation of the upper group of lamps, while a set of terminal contacts at the top of the unit is operatively associated with the lower group of four lamps. The application of successive high-voltage pulses (e.g., 500 to 4,000 volts from, say, a piezoelectric source controlled by the shutter of a camera in which the array is inserted) to the terminal contacts at the lower end of the unit causes the four lamps at the upper half of the array to be sequentially ignited. The array may then be turned end for end and again inserted into the camera in order to flash the remaining four lamps. In this manner, one group of lamps (or half of the array) functions as a flash extender for the other group of lamps, so that only the group of lamps relatively farther from the camera lens axis can be flashed. The purpose of such an arrangement is to position the "active" group of flashlamps farther above the camera lens in order to reduce the possibility of a "red-eye" effect that causes the pupils of a person's eyes to appear red or pink in flash pictures taken when the flashlamp is close to the camera lens.
The flip flash circuit board comprises an insulating sheet of plastic having a pattern of conductive circuit traces, including the terminal contacts, on one side. The flashlamp leads are electrically connected to these circuit traces by means of eyelets secured to the circuit board and crimped to the lead wires. The circuitry on the board includes six printed, normally open, connect switches that chemically change from a high to lower resistance so as to become electrically conducting after exposure to the radiant heat energy from an ignited flashlamp operatively associated therewith. The purpose of these switches is to provide lamp sequencing and one-at-a-time flashing. The four lamps of each group are arranged in a parallel circuit with three of the four lamps being connected in series with their respective thermal connect switches. Initially, only the first of the group of four lamps is connected directly to the voltage pulse source. When this first group flashes, it causes its associated thermal connect switch (which is series connected with the next or second lamp) to become permanently conductive. Because of this action, the second lamp of the group of four is connected to the pulse source. This sequence of events is repeated until all four lamps have been flashed. A problem with such a circuit and other similar arrangements is that if one lamp short-circuits internally upon flashing, the successive parallel lamps of that group of four cannot be flashed; i.e., the remaining lamps are shorted out.
It has been found that this problem can be eliminated by the use of a radiant-energy-activated switch that is normally conducting and which becomes nonconducting subsequent to exposure of the actinic output of the flashlamp associated therewith. This normally closed disconnect switch is used in series with each of the lamps, except the last lamp, in a sequentially flashing parallel group of high voltage flashlamps. It may be used, if desired, in addition to the printed connect switches, which is normally open and which close upon actinic exposure. Hence, the modes of action of these two types of switches are opposite from one another--the disconnect switch interrupts the igniting circuit of the lamp in series with it upon firing of that lamp, while the thermal connect switch establishes the igniting circuit for the next lamp upon being activated.
A particularly fast acting disconnect switch is described in the aformentioned U.S. Pat. No. 4,017,728 of Audesse et al, wherein the switch element comprises a piece of electrically conductive, heat shrinkable, polymeric material which is positioned so as to be in operative relationship with the radiant output from the series connected lamp during the ignition thereof. Each switch element is attached at both ends to the lamp igniting circuit with the midportion of the element being spatially suspended to avoid contact with heat absorbing surfaces. Upon ignition of its associated lamp, the midportion of the switch element rapidly shrinks and separates in response to the radiant output of the lamp, thereby providing a quick-acting, reliable open circuit to high voltages.
The overall construction of the flip flash unit comprises front and back plastic housing members with interlocking means for providing a unitary structure. The front housing member is a rectangular concavity and the back housing is substantially flat. Sandwiched between the front and back housing members, in the order named, are the flashlamps, a unitary member, preferably of aluminum-coated plastic, shaped to provide the eight individual reflectors of the array, an insulating sheet, a printed circuit board, and an indicia sheet, which is provided with information, trademarks, and flash indicators located behind the respective lamps and which change color due to heat and/or light radiation from a flashing lamp, thus indicating at a glance which of the lamps have been flashed and not flashed.
The above-referenced copending application Ser. No. 840,497, Audesse et al, filed Oct. 7, 1977, describes an improved multilamp photoflash unit which more efficiently utilizes a given housing volume and thereby reduces the cost of the unit per flashlamp contained therein. More specifically, a compact lamp arrangement is provided whereby additional lamps are contained in a given volume while maintaining light output performance requirements. In a particular embodiment described, ten lamps are provided in a housing having the same dimensions as the above-discussed eight-lamp flip flash units. This greater compactness is provided by arranging the planar array of lamps in two parallel columns with the tubular envelopes horizontally disposed and with the lamps of one column staggered with respect to the other such that the bases are interdigitated. A pair of reflector panels are aligned with the two columns of lamps and arranged to overlie the lamp lead-in wires and bases.
As may well be appreciated, the above-described compact ten-lamp array imposes a significant challenge with respect to the design and layout of a suitable sequencing circuit on the asocciated printed circuit board. Previous lamp sequencing circuits have used either series or parallel "normally open" (N/O) connect switches and/or parallel "normally closed" (N/C) disconnect switches. For example, see the above-referenced U.S. Pat. No. 4,017,728 (parallel switches) and Ser. No. 840,498, filed Oct. 7, 1977 (series switches). It will be noted that in a given circuit all of each type of switch is either in series or in parallel.
The increase in the number of lamps from eight to ten requires that additional N/C and N/O switches be used. This increase, the position of the lamps, and the desired sequence of flashing forces the use of series N/O switches if the previous switch design is employed. The additive resistance of series N/O switches, however, can cause lamp failures, especially when one switch has a discontinuity. An additional switch (or switches) in series increases the likelihood of this failure mode. An even more significant problem created by the close tolerances is a possible discontinuity in the circuit itself caused by the inherent variation in the placement of the N/C switch. If the first N/C switch of either end of the array is slightly off center, it may project beyond the conductive circuit run. The production process slides the circuit boards across each other and, thus, can wipe the circuit paste off the top of the N/C switches. This can cause an open circuit which acts as a spark gap. It has been found that the lamp immediately following a spark gap will flash, but that each succeeding lamp is more likely to fail due to increasing circuit resistance.